The following information comes from Mr. maro's blog entry.
The Sigma User Group in Japan had their annual end of year get together with some Sigma employees (they always invite somebody from Sigma), and managed to squeeze out the following information regarding the SD15:
* the same Foveon sensor is going to be used, but the other components are being revised
* they are thoroughly looking into anything that has room for improvement, so that's why they cannot announce any release dates
* better firmware, faster FPS, faster speed (unknown if this is referring to the general operation or buffer)
* Sigma is apparently aiming the SD15 to be above the mid-range Nikons and Canons
The bottom line is that the SD15 will look like the mock up, but will be a different camera inside.
Cheers.